1. Technical Field
The present intention relates to an arrangement for decreasing galvanic corrosion between metal components. The term metal components is intended to encompass both components comprising pure metals and components comprising metal alloys. The invention is preferably applied in association with a fixing element, such as a screw or bolt connection, which includes a more noble metal than the metal component or the metal components as the fixing element should be in electrical contact with upon mounting. The invention is particularly advantageous when applied at vehicle components, but may also be used for metal components in other applications where a risk for galvanic corrosion exists.
2. Background of the Invention
Chemical attacks of, among other things, oxygen from the air, on a metal, is often referred to as atmospheric corrosion. Gold and platinum are examples of metals that are most resistant to (noble) atmospheric corrosion, while other metals such as aluminum, chrome and stainless steel alloys are normally become covered by a protective layer in air that prevents further attack. On the other hand, the corrosion products associated with iron do not give such a corrosion protecting oxide layer.
In the case that two different metals are brought in electrical contact with each other in a humid environment, a so-called galvanic corrosion may occur. The effects of so-called galvanic corrosion may be studied for instance by launching a motorboat with a propeller shaft of stainless steel and a propeller of bronze in salt water. In this instance, galvanic corrosion leads to considerable attacks on the lesser noble propeller after only a short time. Consequently, in the case of motorboats, it is customary to attach a so-called sacrifice anode of an even lesser noble metal than those present in the propeller under the water line, which leads to the result that the sacrifice anode corrodes instead of the propeller. Thus, the sacrifice anode gives the propeller a protection against galvanic corrosion until the anode has been consumed and must be replaced by another.
The galvanic corrosion may be explained through a discussion of electrochemical activity. Metals can be divided in a so-called electrolytic electromotive chain. If pieces of two different metals, being in different positions in the electromotive chain, are located in an electrolyte and are in electrical contact with each other, a galvanic element appears. Thereby, the metal at the lowest position in the electromotive chain becomes an anode or solution pole, while the second metal becomes a cathode or precipitant pole.
The location of the metals in the electromotive chain may be stated as so-called normal-electrode potentials at 25xc2x0 C. whereby, for instance, iron (Fe) is often stated to have the value of xe2x88x920.440 volts and magnesium (Mg) a value of xe2x88x922.37 volts. In the case that pieces of magnesium and iron are brought into electrical contact with each other in an electrolyte, a short-circuited galvanic element is obtained, where the magnesium functions as an anode/solution pole and dissolves during formation of ions and delivering electrons. At the iron-piece functioning as cathode/precipitant pole, hydrogen gas (H2) will instead be precipitated out of the electrolyte, or oxygen gas will be reduced.
It should be noted that the humidity that is in air which is normally present contains sufficient dissolved elements to function as an electrolyte. Therefore, galvanic corrosion may appear without the actual metals being immersed in any liquid.
There are a number of methods to protect metals that are susceptible to corrosion against atmospheric corrosion. Amongst such methods, a coating may be applied, for instance, using different anti-corrosive agents, galvanization, and chromium plating etcetera.
Regarding protection against atmospheric corrosion and/or galvanic corrosion, it is also known to use particular sacrifice coatings such as a corrodible first metal that is coated with an even more corrodible second metal with the ability to form a protecting oxide layer. Thus, it is previously known, to coat a metal substrate by means of steam dumping with a sacrificial coating of aluminum to protect the substrate from corrosive attacks.
Further, it is previously known that different cathode surfaces have different kinetic properties regarding disintegrating water and reducing hydrogen ions and oxygen gas.
Further a method is described in U.S. Pat. No. 4,980,195 to prevent atmospheric corrosion of steel, in which the steel substrate, which should be protected, is coated with aluminum by means of ionic steam dumping coating. The steel coated with aluminum is then immersed in an aqueous acid solution of an indium acid salt, which is said to give small surface deposits of metallic indium. These surface deposits are stated to be able to activate xe2x80x9cthe sacrificial propertiesxe2x80x9d of the aluminum of the coating so that the coating provides a better corrosive protection than a pure aluminum coating.
Further, U.S. Pat. No. 4,844,943 describes a process to protect a metallic surface against corrosion depending on the contact with the vanadium and/or sodium oxides at gas combustion temperatures coming up as a consequence of combustion of heavy fuels. The process is stated to include the deposition of a layer of at least one protecting metal chosen from the group containing alkaline earth metals, boron, aluminum, gallium, indium, thallium, silicon, zirconium and titanium on the metal surface. The protective metal is stated to oxidize in situ to form a protective oxide layer reacting with vanadium oxide, sodium oxides or solutions thereof, to form a composition with a softening temperature being higher than the softening temperature of vanadium or sodium oxides.
The International Patent Application No. PCT/DK95/00421 of one of the applicants describes a coated aluminum material containing a substrate of aluminum or aluminum alloy at least with a surface section thereof, and a first layer containing oxides of the substrate material, and a second layer substantially containing indium, tin and/or gallium. The coated aluminum material is particularly useful in cars, aircrafts and buildings, and has very good properties with regard to corrosion resistance, abrasion resistance and low friction.
Among others with purpose to be able to decrease fuel consumption by means of a lower vehicle weight, the use of different lightweight materials has increased even more in the vehicle manufacturing industry. This has lead to an increased use of materials such as aluminum, magnesium and alloys of these metals. For different reasons it is still necessary and/or desirable to be able to use more traditional materials such as steel alloys for certain vehicle components. The increased amounts of different metals and metal alloys, which are used in modern engine vehicles, have lead to an increased risk that galvanic corrosion should appear between different components.
The risk for galvanic corrosion may often be particularly great in association with attachment means or elements, fasten articles or consoles, since, for instance, bolts and screws for reasons of strength often contain a steel alloy, while the components which should be joined together often contain light weight materials. Further, the use of attachment means is often particularly great in positions, for instance of engine vehicles, where a high air humidity provides good prerequisites for galvanic corrosion, such as in association with wheel suspensions of the engine vehicle or inside the engine bay.
In order to reduce the problems of galvanic corrosion of attachment means, for instance for use in engine vehicles, an intermediate aluminum plate is often mounted between, for instance, a bolt or a screw of steel at the attachment means, and a component of a light weight metal or light weight metal alloy. This is particularly common in the case of magnesium alloys.
Even if the solution of such an electrochemical plate is more compatible and it reduces the problem of galvanic corrosion between the attachment means and the actual metal component, it gives an undesirable added weight, an undesirable added cost, a more complicated mounting, and in some cases a greater risk for settlements in the joint.
One purpose of the present invention is to provide an arrangement for decreasing galvanic corrosion between metal components, removing the need for corrosion decreasing plates or spacers, and thus, admits both cost and weight savings, and forms more simple mountings and more stable joints.
This purpose is achieved by way of an arrangement that includes a first component in which a first metal is a part thereof, and a second component in which a second metal is a part thereof. The first metal has a higher normal-electrode potential e0 than the second metal and the first component is intended, after mounting, to be in electrical contact with the second component. In one aspect of the invention, the first component is coated with a substantially continuous surface layer that is adjusted to provide the second component with an insignificant galvanic corrosion velocity after the mounting.
A second purpose of the present invention is to provide an arrangement for decreasing the galvanic corrosion between metal components which may be accomplished with a proportionally low environmental influence regarding both work environment and surrounding ecosystems. In at least one embodiment, this second purpose is accomplished by a surface layer of the arrangement that substantially consists of indium (In), an indium alloy or an indium compound.
Further purposes of the present invention will be apparent from the following description, as will the features that enable these additional purposes to be achieved.